others that do not quite fit neatly into these groups, but that have effects similar to one or more of them, eg venlafaxine, mirtazepine, reboxetine and trazodone.

How do they work?

Antidepressants work by boosting levels of natural chemicals called neurotransmitters that are found in the brain.

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Neurotransmitters are involved in controlling or regulating bodily functions. Two chemicals that are involved in the control and regulation of mood are:

noradrenaline

serotonin.

It's thought depression occurs when the nerve cells in the brain don't release enough of these two chemicals.

On release from the nerve cells these chemicals act to lighten mood.

When the nerve cells reabsorb these chemicals, they no longer have an effect on mood.

Antidepressants work in different ways to increase the levels of these chemicals in the brain to normal.

SSRIs are the newest type of antidepressant. They work by preventing serotonin being absorbed back into the nerve cells. This prolongs the mood-lightening effect of any released serotonin and so helps to relieve depression.

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TCAs and MAOIs are much older classes of antidepressants.

TCAs work by preventing both noradrenaline and serotonin being absorbed back into the nerve cells, with the same mood-lifting effect. Trazodone is related to the TCAs.

MAOIs work by preventing the natural breakdown of serotonin and noradrenaline by a chemical called monoamine oxidase. This leaves more of these chemicals active in the brain and in this way helps relieve depression.

How long do they take to work?

All antidepressants take between two and eight weeks to start having an effect. This means you need to keep taking them, even if they don't seem to make much difference in the beginning.

There is no general consensus on the length of antidepressant treatment that is appropriate for bipolar depression.

Some people can stop the antidepressant as soon as they recover from the depressive episode, but others may have to continue on it longer, maybe even for months. This usually depends on how you have previously responded to antidepressants.

Ideally, the antidepressant should be stopped as soon as the depressive episode has fully resolved, because it can carry a risk of triggering a manic or hypomanic episode. When stopping an antidepressant, the dose should usually be tapered down over a period of four weeks.

People with chronic or recurrent depression may be treated long-term with a low dose of an antidepressant (usually an SSRI) in combination with a mood stabiliser, quetiapine or lamotrigine.

Are they addictive?

No. It is possible for some antidepressants to produce unpleasant symptoms when they are stopped (sometimes called a discontinuation syndrome).

This is not addiction or dependence because these symptoms are:

temporary

do not involve a craving for the medication

can usually be avoided if the drug is tapered off slowly rather than stopped abruptly.

What are the differences between them?

As far as their effectiveness in treating depressive episodes goes, all antidepressants are about as good as each other.

Not everyone responds to the first antidepressant they take, but there are other types available.

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Aside from the way in which they work, the main differences between the groups of antidepressants are in the side effects they can cause. SSRIs are most commonly used in bipolar depression because they are least likely to cause a switch into mania.

While there is not a single medicine that is free from potential side effects, people respond differently to medicines. You could experience none, few or most of the side effects listed.

Some antidepressants have additional actions that may influence their prescription, for example trazodone and venlafaxine also relieve anxiety and certain SSRIs can also be used in panic disorder or obsessive-compulsive disorder.

All antidepressants are licensed to treat depression, but not specifically bipolar depression.

Types of antidepressants

Selective serotonin reuptake inhibitors (SSRIs)

SSRIs have fewer troublesome side effects than the older tricyclics and MAOIs, making them the preferred type of antidepressant for people with bipolar depression.

SSRIs are less likely than other antidepressants to cause abnormally high mood (mania or hypomania) when used to treat depressive episodes in bipolar disorder.

SSRIs are less sedating than the older antidepressants and are more suitable for people with heart problems and those who feel slowed up by their depression.

The most common side effects are gastrointestinal effects such as nausea, vomiting, diarrhoea and constipation.

Other side effects include headache, dizziness, agitation and insomnia.

All antidepressants can be associated with sexual problems such as impotence, but this seems to occur most frequently with the SSRIs.

Fluoxetine may be used less frequently in bipolar depression than other SSRIs because it stays in the body for a long time after treatment is stopped, which could be a problem if treatment needs to be stopped quickly because of a manic episode.

Tricyclic antidepressants

Tricyclic antidepressants are only rarely used in bipolar depression because they carry the highest risk of causing sudden switches to mania.

All TCAs cause drowsiness to varying degrees. Amitriptyline and dothiepin are the most sedating and these may be of benefit to people who are also anxious or agitated. Imipramine and lofepramine are less sedating.

Other common side effects include constipation, difficulty in urinating, blurred vision, dry mouth and weight gain.

People with heart disease should not take TCAs.

Venlafaxine works in a similar way to the TCAs, but does not produce the side effects associated with these antidepressants. However, it also seems to carry a higher risk of causing sudden switches into mania.

Monoamine oxidase inhibitors (MAOIs)

In general, MAOIs are used far less frequently than the other antidepressants because they interact with certain foods and require strict dietary restrictions.

MAOIs can also cause severe adverse reactions if taken with many other medicines, including some over-the-counter cough and cold remedies.

Moclobemide is a newer MAOI and is used more frequently than the older MAOIs. It is believed to cause fewer problems than traditional MAOIs, but caution is still required with certain foods and medicines.

The materials in this web site are in no way intended to replace the professional medical care, advice, diagnosis or treatment of a doctor. The web site does not have answers to all problems. Answers to specific problems may not apply to everyone. If you notice medical symptoms or feel ill, you should consult your doctor - for further information see our Terms and conditions.

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